At present, it is multiple-deck screening machines that are most commonly used in practice. In these devices, the sizing of polydisperse materials is carried out by letting them pass through a number of screens with differently sized meshes disposed one beneath the other. One of the parameters controlling the separation process is the amplitude acceleration which is known to depend on the amplitude and frequency of vibrations. For efficient separation of the material using these designs of the screening machines, it is necessary to assign to each screen vibrations with an optimum amplitude acceleration depending on the manufacturing parameters, the most important of them being the separation size.
Known in the art is a vibratory screening machine of WA series manufactured by "Rhewum" of the Federal Republic of Germany (Aufbereitungs-Technik, Nr.7, July 1977, (G.Erlenstadt, "Schallsiebmaschinen-Weiterentwicklung und neue Betriebsergebnisse", p.p.333-336) comprising a frame, screens with meshes of different size arranged within the frame one above the other, pushers formed by impact levers located beneath the screen, each of them being mounted on a supporting shaft and adapted to make contact with the surface of the screen, and electromagnetic vibrating drives. The electromagnetic vibrating drives are coupled each with one supporting shaft for reversible rotation thereof through a given angle. The impact lever secured to the shaft then transmits vibrations of a given amplitude from the vibrating drive to the overlying screen.
A disadvantage of this device is its high cost due to a plurality of expensive electromagnetic vibrating drives, their number increasing with the number of screens in the screening machine.
Known in the art is a vibratory screening machine (DE, C, 1239919) comprising a frame, at least two screens arranged in the frame one above the other, pushers formed by double-arm levers and located in the interscreen space, each of them being rigidly secured to the shaft and adapted to make contact with different screens, and a vibrating drive. The vibrating drive is connected with one of the shafts for reversible rotation thereof through a given angle, while the other shafts are coupled to this shaft and to each other by means of kinematic transmissions. Each push lever has arms of equal length arranged symmetrically about the shaft axis. These pushers transmit synchronous vibrations of a particular amplitude from the vibrating drive to the screens.
In the device mentioned above, the vibrations imparted to the screens have an equal amplitude acceleration defined by the amplitude-frequency characteristic of the vibrating drive. Such screen vibrations fail to provide the required efficiency of screening the material because of different processing conditions the material is subjected to on each individual screen.